1. Field Of The Invention
The present invention concerns raster type cathode ray tube displays particularly with respect to full field memory refresh apparatus to provide the symbology to be displayed.
2. Description Of The Prior Art
Prior art raster type CRT displays traditionally utilize two full field refresh memories in which one of the memories is updated with symbology utilizing a vector generator while the other memory is scanned for display. The roles of the two memories are reversed on alternate display cycles in "ping-pong" fashion. This prior art arrangement has the disadvantage of requiring excessive apparatus and power particularly when multiple channels of raster symbology are required and is often referred to as a double buffered system. In the double buffered configuration the excessive circuitry, space and power requirements result in excessive equipment cost. In such double buffered systems there is a one to one correspondence between the storage cells of each full field memory and the pixel resolution elements of the display screen. In the multiple channel configuration two full field memories are required for each channel exacerbating the circuitry, space, power and cost disadvantages.
An alternative prior art configuration is the utilization of a single full field memory updated by the vector generator during the vertical retrace period. This arrangement has the disadvantage that in typical calligraphic combination raster and stroke systems the vector generator is utilized to write stroke symbology on the display during the vertical retrace period and is thus unavailable for refresh memeory updating.
Prior art raster display systems often operate in a plurality of line rate TV modes, for example, 525 and 875 line rates. Plural line rate modes further complicate the addressing of the full field memory in such systems.